Aspects of the invention relates to transparent conductive films consisting of an oxide of indium and tin (ITO) and processes for forming them, particularly to transparent conductive films with very low resistivity formed on organic substrates without heating the substrates and processes for forming them.
Indium tin oxide (ITO) is widely used for transparent electrodes in various display devices such as liquid crystal displays and organic EL displays as well as transparent electrodes in touch panels.
ITO transparent conductive films are usually formed on glass substrates and provided with low resistivity by heating the substrates. For example, an ITO transparent conductive film with high crystallinity and low resistivity can be prepared by forming an ITO film by reactive DC magnetron sputtering using a sintered ITO as a target while heating the substrate to a temperature of 300° C. or more. Alternatively, an ITO transparent conductive film with low resistivity can be prepared by using an In—Sn alloy as a target and depositing a suboxide of In, InO and Sn on a substrate by reactive DC magnetron sputtering followed by a heat treatment at 170-180° C. in the atmosphere.
Thus, ITO must be crystallized and the substrate must be heated to at least 160° C. or more in order to obtain a transparent conductive film having a resistivity of 10−3 ohm.cm or less, e.g. on the order of 10−4 ohm.cm. If an ITO film is deposited without heating the substrate, the film shows a mixed amorphous/microcrystalline phase with a high resistivity on the order of 10−3 ohm.cm.
In the field of liquid crystal displays and organic EL displays, it has been proposed to replace glass substrates by organic substrates such as plastic substrates or plastic films, taking into account weight, thickness reduction and ability to be handled. Transparent electrodes for touch panels must also be formed on plastic films. However, it is difficult to heat these organic substrates such as plastic substrates and plastic films because they are vulnerable to heat. Thus, a major issue is to find a method for forming an ITO film with low resistivity on a heat-vulnerable organic substrate.
A method for forming an ITO transparent conductive film on a plastic film by reactive sputtering applying a bias voltage has been proposed (see e.g. JPA SHO 62-222518). According to this method described in publication document 1, an ITO transparent conductive film is formed on a transparent plastic film substrate by reactive sputtering while applying a bias voltage between the transparent conductive film and the ground for accelerating ions in the direction of the substrate, whereby a transparent conductive film with high transparency and adhesion and improved wear resistance can be stably prepared.
The so-called gas flow sputtering has been proposed as a method for forming various films without damaging substrates (see e.g. JPB HEI 2-14427 and JPA 2001-140066). The gas flow sputtering method is a transport sputtering method wherein sputtered particles generated in the anode are transported by an atmospheric gas flow toward the substrate, and JPA 2001-140066 discloses a method for forming a thin film by providing a exhaust port for discharging a sputtering gas in the direction perpendicular to the center axis of the target and placing a substrate on the side of the exhaust port in order to protect the substrate against the influence of plasma and high-energy particles.
However, the method of JPA SHO 62-222518 is essentially an extension of the prior art described above, and the resulting transparent conductive film does not have a low enough resistivity because the substrate is not heated, and therefore, it is difficult to achieve a transparent conductive film having a resistivity of, e.g., 10−3 ohm.cm or less.
In the gas flow sputtering method disclosed in patent JPB HEI 2-14427 or in JPA 2001-140066, the protection of the substrate against damages is stressed, but the characteristics of the deposited films are not always sufficiently considered. Especially, there is no report about the application of gas flow sputtering for forming an ITO transparent conductive film, or about the film characteristics of the deposited ITO transparent conductive films. In fact, the inventors tried to form an ITO transparent conductive film by the method described in patent JPA SHO 62-222518 or in JPB HEI 2-14427, but failed to deposit an ITO transparent conductive film on the order of 10−4 ohm.cm.
The present invention was proposed in view of the actual situation of the art for the purpose of providing a transparent conductive film having an unprecedented low resistivity formed on an organic substrate without heating the substrate as well as a process for forming it.